1. Field of the Invention
The present invention relates to a ceramic glow plug to be fitted to a diesel engine.
2. Description of the Related Art
A ceramic glow plug generally comprises: a metallic sheath; a cylindrical main metallic shell having at the front end thereof a holding part which extends inward and holds a rear part of the metallic sheath; a ceramic heater, a terminal electrode inserted into the cylindrical main metallic shell on its rear end side and insulated therefrom; and a pair of external connecting wires connected to the lead-out wires in such a manner that one side ends of the external connecting wires are brazed respectively to the exposed areas of the lead-out wires and the other ends thereof are electrically connected respectively to the main metallic shell and the terminal electrode.
This ceramic glow plug is produced through the following steps (1) to (4).
(1) A heater main body comprising a heating material and a pair of lead-out wires having one-side ends connected respectively to the ends of the heating material is embedded in a powder of a ceramic, e.g., Si.sub.3 N.sub.4, and this powder containing the heater main body embedded therein is sintered by hot pressing to produce a ceramic heater.
(2) One-side ends of two external connecting wires are brazed respectively to exposed areas of the lead-out wires, before the ceramic heater is inserted into and fixed to a metallic sheath.
(3) This assembly is inserted into a cylindrical main metallic shell, and a rear part of the metallic sheath is brazed to the inner wall of a holding part of the main metallic shell.
(4) A terminal electrode is fixed to the main metallic shell with an insulator and a nut.
However, the ceramic glow plug produced through the steps described above has the following problems.
During brazing, those areas of the lead-out wires which are exposed on the bake surface may suffer oxidative corrosion due to the brazing temperature of 800 to 1,100.degree. C.
In this case, the lead-out wires corrode at an increased rate during use of the ceramic glow plug. Further, in such a ceramic glow plug, irregularity in initial resistance is increased and change in resistance during ordinary use is also increased.